Nearly a year and a half after landing on opposite sides of the planet, NASA's two Mars Exploration Rovers have survived far beyond the three months that their designers had counted on. Now mission planners wonder if the rovers can keep on trekking for another year or more.

They'll have to avoid pitfalls like the soft dune that mired Opportunity until June 3. After traveling 3.32 miles (5.34 kilometers) on Mars and crossing many similar dunes, Opportunity got stuck April 26 when her wheels sank nearly to their axles in soft soil. For three of those five weeks, she spun her wheels, making only an inch or so a day, before she escaped. (See an animation of one of her spinning wheels). "Thank goodness it's free," says Joy Crisp, the mission's project scientist at the Jet Propulsion Laboratory in Pasadena, California. "We will have to proceed much more slowly than we did before."

Mission managers plan to have Opportunity spend a while studying the rover-trapping dune—dubbed Purgatory Dune—for clues that might warn them before the machine drives into another such predicament. Then continuing, they will send it south and deeper into a mysterious region of "etched terrain." From orbit, this region looked as if its surface had been eaten away and roughened, perhaps by acidic waters, which could mean that many more hazards are ahead for the rover. But Opportunity's first glimpses of the etched region suggest that what had looked like rough terrain might be easier to traverse than had been feared. Ultimately scientists hope to cover another couple of miles to a half-mile-wide crater called Victoria, six times bigger than any the rover has explored so far.

Meanwhile the sister rover Spirit, climbing the east flank of Husband Hill in the Columbia Hills, has taken some of the most unusual pictures ever returned from another planet: whirlwinds dancing along the flat plain of Gusev Crater. NASA scientists pieced many of the frames into somewhat jerky but entertaining movies. About 160 feet (50 meters) above the base of the hills, Spirit is beginning to see layered bedrock that preserves enough detail for geologists to begin piecing together the hills' history. The leading hypothesis is that they formed billions of years ago, when a series of explosive volcanic eruptions dumped piles of ash and debris.

The months on Mars have been kind to both rovers. A balky wheel on Spirit seems to have recovered from whatever ailed it. One of the biggest sign of age: Spirit rover now has a dull "rat," the rock abrasion tools that grind into boulders and bedrock with diamond-studded wheels. It was designed to last for three rocks' worth of use, but instead has gotten many times that much work.

Faced with the rovers' remarkable endurance, NASA in April approved an extension of the mission through September next year—doubling the length of what has already been a longer and more fruitful mission than anyone had hoped.